Transformation produces many changes in cell physiology. To study these changes we developed a new ultrastructural immunocytochemical localization technique which allows electron microscopic antibody localization to intracellular protein in the presence of well preserved morphology in cultured cells (EGS procedure). Using this technique, we have localized the contractile proteins actin, myosin, and filamin, and the structural proteins of 100 A filaments, microtubules (tubulin), and the bristle coat of coated endocytic vesicles. We have also localized the src gene product polypeptide p60src of SR-avian sarcoma virus transformed cells, and found it associated with the inner aspect of the plasma membranes, concentrated at surface ruffles and gap junctions.